Semenya practically jogging to an 800-meter win at a world-class Diamond League meet, 2016. Getty Images.

South African sports scientist Ross Tucker and transgender medical physicist Joanna Harper have written a fascinating piece on women’s sports, and the current lack of any limits on testosterone levels in female track and field athletes. This issue is likely to be one of the most controversial and important discussions at this year’s Rio Olympics.

A year ago, the Court of Arbitration for Sport ruled to allow hyperandrogenous women to compete in track and field without suppressing their testosterone levels. Hyperandrogeny and intersex are conditions commonly referred to as disorders of sexual development. That ruling allows track’s governing body, the IAAF, two years to prove that testosterone provides an unfair advantage when produced naturally, as it is in hyperandrogenous women.

I wrote at the time that this wasn’t a decision for fairness because the CAS grabbed the feel-good, easy case of Indian sprinter Dutee Chand—a mediocre athlete who had naturally elevated testosterone levels—while ignoring the detrimental ramifications of their decision.

The basis for the existence of separate men’s and women’s divisions in sports is that athletes should, due to physical differences between the sexes, be competing against opponents of their own sex. The neat binary implied by the existence of men’s and women’s division, though, is a convenient fiction. And any dividing line—especially when things like gender identity get involved—immediately becomes very messy. Unlike a weight class in boxing that’s easy to defend—weight is objective and quantifiable—making a line between men and women where none exists in nature is hard to draw, but essential to defend.

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In the past, the attempt to define athletes as women and therefore eligible to compete in women’s divisions led to grotesque consequences. Female athletes were forced to parade nude in front of judges, or undergo horribly invasive gender testing. But while these methods have rightly been abolished, the larger question remains: how do you define who should and who should not be able to compete in women’s athletics?

In 2011, motivated by a scorching 1:55 800-meter performance by 18-year-old South African Caster Semenya, scientists and physicians at the International Association of Athletics Federations abandoned attempts to determine gender, and instead set an upper limit on the testosterone level of women who were allowed to compete. This limit was set three-fold above the testosterone levels observed in 99 percent of women, and in fact the 10 nmol/L limit is in the low to normal range for men. It was reasoned that the very few women who naturally produce T levels higher than that would have an unfair advantage, and would therefore be required to either surgically or chemically suppress their testosterone levels, or not compete.

This solution, while far from perfect, was as fair and inclusive as any method devised thus far. It was in place from 2011 through 2015, until Chand brought her case to the CAS, which ruled in her favor and abolished any testosterone limit for women. To use a wrestling analogy, there are still weight classes but there’s nothing to stop a 170-pound wrestler from competing in the 145-pound class.

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The ruling has only been in place for nine months, but already in that period four hyperandrogenous women have posted spectacular times in one event—800 meters—and seem likely to make up four of the eight spots in the Olympic final of the women’s 800.

Track and field is currently the sport embroiled most deeply in this incredibly complex, fraught, fascinating, and far-reaching topic, which is why I recommend anyone interested read Tucker and Harper’s full treatise. I’ve pulled out some of the more noteworthy passages below.

The Gender Lightning Rod

Tucker begins by providing background from 2011, when the IOC and IAAF first limited women’s testosterone levels, and how the limit affected the performance of Caster Semenya, who began suppressing her testosterone levels to below the limit in order to compete (emphasis his):

Semenya’s performances, under this policy of reducing testosterone, dropped off in a predictable manner. Having run the 1:55.45 at [age] 18, she never got close again, though did win Olympic silver in London (behind a doper), and a World silver in 2011. Last year, she failed to advance beyond the semi-finals in Beijing, and hadn’t even made the qualification mark for the preceding year’s Commonwealth Games. 2:00 had become a significant barrier, when the world record had been plausible at 18.

Now, she is untouchable. People will (and have said) that it’s down to her focused training, recovery from injury and so forth, but I’m not buying that. The change has happened for an obvious reason – the restoration of testosterone levels, and that is thanks to the courts – CAS, the Court of Arbitration for sport, last year ruled that the IAAF could no longer enforce the upper limit of testosterone, and in so doing, cleared the way for Semenya, and at least a handful of others, to return to the advantages that this hormone clearly provides an individual. That CAS ruled this way because they felt that there was insufficient evidence for the performance benefits is one of the stupidest, most bemusing legal/scientific decisions ever made.

In any event, the situation now is this – Semenya, plus a few others, have no restriction. It has utterly transformed Semenya from an athlete who was struggling to run 2:01 to someone who is tactically running 1:56. My impression, having seen her live and now in the Diamond League, is that she could run 1:52, and if she wanted to, would run a low 48s 400m and win that gold in Rio too.

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Joanna Harper is transgender, and a medical physicist at Providence Portland Medical Center in Oregon and 2:23 marathoner who competes with chemically-achieved lower testosterone and higher estrogen levels. She has authored the only study (PDF warning) on transgender athletic performance, and advises the IOC on gender topics.

Unlike many in the transgender community, Harper came down strongly against the CAS ban on testosterone limits for women, partly based on her own transition experience. When she started hormone replacement therapy to reduce her testosterone levels and increase estrogen, her running performance almost immediately dropped by 10% to 12%, which is about the average difference between men’s and women’s performances:

While human rights advocates are deliriously happy over the CAS ruling, those who love women’s sport are mortified. Those Intersex athletes who previously used medications to reduce their T are now off of those medications, and are running faster. Allowing these athletes to compete in women’s sport with their serious testosterone-based advantage threatens the very fabric of women’s sport.

I would further suggest that, while it might not be a right, success in sports is one of the greatest advancements in women’s lives. If we value women’s equality, it is imperative that we protect the ability of all women to succeed in sports.

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Harper supports a return to the policy that was in place between 2011 and last year’s CAS decision—testosterone limits of 10 nmol/L—meaning intersex or hyperandrogenic women would need to lower their testosterone levels to within that range in order to compete.

I believe that billions of potential female athletes deserve the right to compete with some semblance of a level playing field, and that requiring all women to compete within a given testosterone range is the best way we currently have to create such a playing field.

As it is now, with no T limits, Harper describes a very possible scenario based on the fact that, though hyperandrogenism is very rare, it provides such an extreme advantage such that, very quickly, individuals with this advantage rise to the top. She explains how this rare condition (1 in 20,000 in the general population) can be disproportionately represented at track’s elite levels.

The DSD [disorders of sexual development] that probably imparts the largest athletic advantage is called 5-alpha reductase deficiency or 5-ARD. Children born with 5-ARD have a Y chromosome, but have a deficiency in the enzyme that is used to convert testosterone to dihydrotestosterone or DHT. In turn, DHT is responsible for the development of external male genitalia, hence babies with 5-ARD are often assigned female gender at birth. After puberty, girls with 5-ARD have T in the low-normal male range, and hence have a huge athletic advantage over other women.

5-ARD is extremely rare in the general population, but there are isolated pockets around the world where it is not uncommon. 5-ARD is an autosomal recessive condition, and so both parents must carry the gene for it in order for a child to be affected. In those remote areas where 5-ARD and consanguinity (inbreeding) are both common, a significant percentage of the population will carry the gene for 5-ARD. Given the globalization of sport, it is possible that those interested in developing the next generation of women’s sports stars will look to these areas to find girls with 5-ARD, and aid in their athletic progress.

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The argument is frequently proffered that all elite athletes benefit from some sort of rare genetic condition and are not required to limit its advantage. Tucker refutes that argument by suggesting if a category is identified—say, weight class in boxing—its defining limits need to be defended. While height in basketball is an advantage, there are no height categories in basketball. Here is Tucker’s last word on that (emphasis his):

And let me be very clear – this is not the same as tall people dominating in basketball, or people with fast-twitch fibres dominating sprints. We do not compete in categories of height, because we have decided that there is no need to “protect” short people. We certainly do not compete in categories of muscle biochemistry or neurology.

There are many aspects and arguments in this debate, and I respect most of them, but this particular offering of “whataboutery” is garbage, utterly inadmissible in this complex debate. If you want to play whataboutery in this way, think about weight classes in boxing, contact sports, rowing. Would it be fair if someone said “I can’t help my physiology, and I’m 2kg over the limit for “lightweight”, so let me in?” Or, if you did create a division for height in basketball, should we allow people who can’t help that they’re tall because of genes to come down and play with those under 6 foot? Of course not.

Point is, if you create a division to ensure performance equality based on a known performance advantage, then you absolutely must defend that division, however ‘arbitrary’ the line appears to be. The division between men and women is clear. It is obviously significantly influenced by testosterone, and few physiological variables are as clearly (if imperfectly) separate like testosterone is. If that division is to be respected, as it should, then hyperandrogenic women should have some regulation in place.

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Even though testosterone limits, or lack thereof, affect very few people, the impact on women’s individual sports, like track, at the highest level is huge. Harper described how a lack of testosterone limit could play out at the Rio Olympics, only a year after the non-policy was instated:

While Caster Semenya has gotten most of the media attention, she is far from the only presumably intersex athlete to have competed at a very high level in athletics. In fact two of the three medalists in the 800 meter race at the recent indoor world championships are probably intersex. It is very possible that we could see an all intersex podium in the 800 in Rio, and I wouldn’t be surprised to see as many as five intersex women in the eight-person final. There are potential intersex medalists in other running events too. The mutations that we are talking about are very rare, and these women are hugely over-represented.

Looking beyond Rio, Harper described two damaging scenarios if the current non-policy is allowed to play out over years and decades. One is that profiteers looking to cultivate the next women’s sports stars would identify and exploit populations in which the genetic condition for hyperandrogenous girls is most likely.

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The second is that some transgender women—most of whom “desperately want to lower their T levels”—would be willing to compete against women without altering their testosterone levels.“This would be a nightmare, not only for the world’s female athletes, but also for those trying to increase acceptance for trans people everywhere,” writes Harper.

An Abundance of Circumstantial Evidence

The current CAS ban on testosterone limits is a temporary measure. The IAAF was given two years from August 2015 to provide evidence that testosterone above 10 nmol/L provides unfair advantage to women. While Semenya has demonstrated a brilliant, if results-only, example of this advantage, providing scientifically documented proof that is statistically significant will be almost impossible, not least because of the difficulty inherent in doing so in an ethical manner.

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The scientists and physicians that worked with the IAAF to arrive at the 10 nmol/L testosterone limit found that hyperandrogenism was 140 times more prevalent in elite female athletes than the general population.

“I’d call that pretty strong evidence for some advantage,” said Tucker. “The alternative is that they start off the same but the process of training elevates their testosterone to what are basically pathological levels, and that’s difficult to believe or to find evidence for. So in my opinion, the evidence is already there, but maybe needs to be shored up and increased to three or four times this size: You probably need to find 50 cases, which means testing maybe 8,000 athletes, and then you’d have compelling circumstantial evidence.”

Testing 8,000 elite female athletes isn’t very likely to be done, and the alternative tests are clearly unethical.

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“You’d have to give a group of women testosterone and see how they improve, which is, obviously, ethically dodgy,” Tucker continued. “There is another way, and this is the study that I would do if it wasn’t also ethically dubious. It’s to identify those women who have disorders of sexual development [such as hyperandrogenism], suppress their endogenous testosterone, and see how they slow down. Then remove the suppression, and watch how much faster they get. Caster Semenya may be a real life example of that happening.”

Tucker made that assertion back in 2015, and Semenya has indeed proved his point in just nine months. But even though she’s dramatically shown the advantage testosterone affords, because it’s not part of a controlled scientific investigation, Semenya’s example remains circumstantial. The very quick appearance of women running times that women, during the T-limiting years, have not approached, is the only compelling, but again circumstantial, evidence available.

Harper said she is working with the IAAF to try to reverse the CAS ban and reinstate testosterone limits for women.

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What’s Next?

As it stands, Caster Semenya is the overwhelming favorite to win gold in Rio, and has a good shot at crushing the world record—maybe in the 400 and the 800. Semenya, and other hyperandrogenous women, will likely be subjected to Olympic levels of speculation and blame rather than the adulation most medal winners receive, and women running 1:58—a time that has, in the era of testosterone limits, been competitive—may not even make the final in the 800.

But in the upside-down world of unintended consequences, Caster Semenya covering two laps in something mindblowing like 1:52 and a podium full of hyperandrogenous women could be the most eloquent argument yet for reinstating testosterone limits. It could be the best thing to happen to women’s sports.